Make the backtrace-debuginfo test less error prone
Make the backtrace-debuginfo test less error prone

LLVM might perform tail merging on the calls that initiate the unwinding
process which breaks debuginfo and therefore this test. Since tail
merging is guaranteed to break debuginfo, it should be disabled for this
test.

This allows us to restore a testcase that I had to remove earlier
because of the same problem, because back then I didn't realize that
disabling tail merging was an option.

cc #27619

aidancully/rust,dwillmer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,pshc/rust,GBGamer/rust,graydon/rust,pshc/rust,pshc/rust,GBGamer/rust,pshc/rust,dwillmer/rust,dwillmer/rust,graydon/rust,dwillmer/rust,GBGamer/rust,aidancully/rust,dwillmer/rust,graydon/rust,graydon/rust,graydon/rust,pshc/rust,aidancully/rust,dwillmer/rust,graydon/rust,GBGamer/rust,dwillmer/rust,pshc/rust,pshc/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,pshc/rust,dwillmer/rust